Laguna Mar Chiquita, a highly variable closed saline lake located in the Pampean plains of central Argentina, is presently the largest saline lake in South America ( 6000 km 2 ). Recent variations in its hydrological budget have produced dry and wet intervals that resulted in distinctive lake level fluctuations. Results of a multiproxy study of a set of sedimentary cores indicate that the system has clearly recorded these hydrological variations from the end of the Little Ice Age ( ad 1770) to the present. Sedimentological and geochemical data combined with a robust chronology based on 210 Pb profiles and historical data provide the framework for a sedimentary model of a lacustrine basin with highly variable water depth and salinity. Lake level drops and concurrent increases in salinity promoted the development of gypsum-calcite-halite layers and a marked decrease in primary productivity. The deposits of these dry stages are evaporite-bearing sediments with a low organic matter content. Conversely, highstands are recorded as diatomaceous organic matter-rich muds. Average bulk sediment accumulation rose from 0AE22 g cm )2 year )1 in lowstands to 0AE32 g cm )2 year )1 during highstands. These results show that Laguna Mar Chiquita is a good sensor of high-and low-frequency changes in the recent hydrological budget and, therefore, document climatic changes at middle latitudes in south-eastern South America. Dry conditions were mostly dominant until the last quarter of the twentieth century, when a humid interval without precedent during the last 240 years of the lake's recorded history started. Thus, it is an ideal system to model sedimentary and geochemical response to environmental changes in a saline lacustrine basin.
Laguna Mar Chiquita, a highly variable closed saline lake located in the Pampean plains of cetral Argentina, is presently the largest saline lake in South America ("-6000km2). The availability of hitorical, instrumental lake-level and salinity data for the past 100 years allows for the calibration of the isotopic archive recorded in the lake sediments. Prolonged intervals with either negative or positive hydrlogical balances have severely modified lakewater levels, salinity and primary productivity, and have also controlled the isotopic composition of both the authigenic carbonate (&8Ooarb and f133Ccarb) and sedimetary organic matter (6"C0,,). Extensive evaporation during lowstand stages results in an enrichment of 180 and 13C in the lake waters, and is recorded in the sediments as the most positive &8O8arb and 613C carb compositions (0.0%o and-1.9Yoo, respectively). Conversely, more negative 6f'8Ocarb and 813Ccarb values (-1.8%o and-3.8Yoo, respectively) are the result of increasing freshwater input into the lake system. The fi3Com values are related to the isotopic composition of the dissolved inorganic carbon pool and the cabonate equilibrium of the lake water. Relatively low 613COm values correspond with high lake levels, low salinity, low alkalinity and high lake productivity. High salinity during lowstands diminishes the amount of primary production and the 613Com value is correspondingly high. The calibrated isotopic model was extrapolated to reconstruct precipitation-evaporation variability from the end of the‘Little Ice Age’ (c. AD 1770) to the present. Low water levels predominated until the last quarter of the twentieth century, when a positive hydrological balance without equivalent in the previous history of Laguna Mar Chiquita became dominant.
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